Pulsed laser polymerization of methyl methacrylate in ionic liquids

1
Pulsed laser polymerization of methyl
methacrylate in ionic liquids
Simon Harrisson
Boston August 2002
2
Ionic Liquids

• Salts that are liquid at room temperature.
• First prepared in 1914 (ethylammonium nitrate).
• Central feature
• Bulky asymmetric cation e.g. C5H5NR, NR4,
  PR4,N-alkyl methylimidazolium

3
Ionic Liquids Green Solvents

• Near-zero vapor pressure.
• Immiscible with many solvents.
• Easily modifiable
• wide range of properties.
• Liquid range of up to 300 C

4
Ionic Liquids Reaction Media

• Many reactions show improved rates and
  selectivities.
• Polymerizations of MMA in bmimPF6 proceed
  much faster than in typical solvents, and to
  higher molecular weights.

5
Polymerization Kinetics

• Rate of polymerization is proportional to
  kp/kt2.
• To understand polymerization in MMA, need to
  separate propagation and termination reactions.

6
Pulsed Laser Polymerization (PLP)
dark time, td
3. Final chain length distribution has a maximum
corresponding to one dark time, with overtones
where chains have survived for more than one
pulse.
1. Laser pulse produces burst of radicals which
initiate chains. These grow during dark time.
2. Subsequent laser pulse terminates most chains,
initiates new ones. Some chains survive.
7
Pulsed Laser Polymerization (PLP)

• Allows measurement of kp and estimation of kt.
• kp
• kpMtd 1
• kt
• DPw Rp
• kp2/ktm M2 (3-?) 2

?
1 Olaj, O. F. Bitai, I. Gleixner, G.
Makromol. Chem. 1985, 186, 2569. 2 Olaj, O.
F. Kornherr, A. Zifferer, G. Macromol. Rapid.
Commun. 1997, 18, 997.
8
Results
9
Termination
Rate of termination decreases as IL increases.
kt ? 1/viscosity.
10
Termination
Rate of termination decreases as IL increases.
kt ? 1/viscosity.
Termination is diffusion-controlled.
Termination
11
Propagation

• kp increases with increasing IL at 50 vol kp
  has doubled.
• Possible Causes
• Preferential solvation.
• Monomer-IL complex.
• Radical-IL complex.
• Solvent effects on transition state.

12
Preferential Solvation?
Is local M increased in vicinity of chain
end? To explain results, M must equal Mbulk
even at 60 IL. Preferential solvation may play
a role, but is unlikely to be the defining factor.
kp.M vs IL
13
Complex Formation?

• Arrhenius parameters (25 - 60 C)
• No effect on A, decreased activation energy (22
  ? 20 kJ.mol-1).
• Compatible with
• Radical-IL complex.
• Solvent effects on transition state.

IUPAC benchmark
bulk
20
50
Arrhenius parameters
14
Effects on Transition State?
Radical-IL complex should decrease kp by
stabilizing radical, unless compensating effects
in TS. Charge transfer states may be more
important in ionic liquids.
Transition state
15
Effects on Transition State?
Radical-IL complex should decrease kp by
stabilizing radical, unless compensating effects
in TS. Charge transfer states may be more
important in ionic liquids.





Transition state
16
Effects on Transition State?
Radical-IL complex should decrease kp by
stabilizing radical, unless compensating effects
in TS. Charge transfer states may be more
important in ionic liquids.





Transition state
17
Conclusions
Rate of MMA polymerization is greatly increased
in ionic liquids. This is due to both increase in
kp and decrease in kt. Decrease in kt is due to
the increase in viscosity of the
medium. Increase in kp may be due to charge
transfer in the transition state or formation
of radical-IL complexes.
18
Acknowledgements
Professor Dave Haddleton Dr Stuart Mackenzie Mr
Richard Doyle Funding from EPSRC Warwick Polymer
Group
19
(No Transcript)